1. Field of the Invention
The present invention relates to thin film type insulated gate semiconductor devices formed on various types of insulating substrates, for example, glass substrates, silicon wafers having an insulating film formed thereon. More specifically, the present invention relates to thin film transistors (TFTs) or thin film diodes (TFDs) and integrated circuits using these thin film devices, for example, active matrix electro-optical devices such as liquid crystal devices. The present invention further relates to a manufacturing method of these devices. In particular, the present invention relates to a low temperature process in which the highest process temperature is preferably not higher than 700xc2x0 C., more preferably 650xc2x0 C. or lower.
2. Description of the Prior Art
Semiconductor devices which have TFT on an insulating substrate such as a glass, such as active liquid crystal display devices and image sensors in which TFTs are used to drive picture elements for example, have been developed in recent years. Glass substrates which have a strain point of not more than 750xc2x0 C., and typically of 550-680xc2x0 C., are generally used for these substrates in view of both mass productivity and cost. Hence, the highest process temperature when such glass substrates are used must be not more than 700xc2x0 C.
Thin film-like silicon semiconductors have generally been used for the TFTs. The thin film silicon semiconductors can be broadly classified into two types, namely those consisting of an amorphous silicon semiconductor (a-Si) and those consisting of a silicon semiconductor which has crystallinity. The amorphous silicon semiconductors can be manufactured comparatively easily through a vapor phase method with a low production temperature, and they are suitable for mass production. Therefore, the amorphous semiconductors are used most generally, but their properties, such as their electric field effect mobility and electrical conductivity for example, are poor when compared with those of silicon semiconductors which have crystallinity. Therefore, there is a considerable demand for the establishment of a method for the manufacture of TFTs using silicon semiconductors which have crystallinity for attaining high speed characteristics.
The characteristics of the gate insulating film are not a serious problem in the case of a TFT where amorphous silicon which has a small mobility has been used. For example, a silicon nitride film which has poor electrical characteristics when compared with silicon oxide can be used for the gate insulating film of a TFT in which amorphous silicon has been used. However, with a TFT in which a crystalline silicon film which has a high mobility is used, the characteristics of gate insulating films are very important as well as the characteristics of silicon films.
The demand for good quality gate insulating films has become very great, especially in view of the improvement in the technology for obtaining crystalline. silicon films. In this connection, with a TFT having a crystalline silicon film in which the channel forming region is comprised of essentially one single crystal or a plurality of crystals and the orientations of all of the crystals are the same (such crystalline forms are known as a mono-domain), the existence of the grain boundaries hardly affects the characteristics of the device, unlike with the usual TFT in which the polycrystalline silicon is used, and the electrical characteristics are determined almost entirely by the characteristics of the gate insulating film.
More specifically, the crystal orientations of two crystals which form a grain boundary are different from one another in the usual polycrystalline structure and, as a result, a high grain boundary barrier is produced. However, even though it is comprised of a plurality of crystals, in a mono-domain structure the crystal orientations of the two crystals which form a grain boundary corresponding to a grain boundary in the usual polycrystalline material are the same and so the barrier at such a boundary is very low, and it is no different from a single crystal. Consequently, in a mono-domain structure the contribution of the grain boundaries to the TFT characteristics is very small, and the characteristics are determined mostly by the gate insulating film.
Thermal oxide films are known as excellent gate insulating films which are ideal for such a purpose. For example, gate insulating films can be obtained using the thermal oxidation method provided that they are on a substrate such as a quartz substrate which can withstand high temperatures. (For example, JP-B-H3-71793) (The term xe2x80x9cJP-Bxe2x80x9d as used herein signifies an examined Japanese patent publicationxe2x80x9d)
Thermal oxide films have very few defects which act as traps when charges such as hot electrons, for example, are implanted and so there is little deterioration in their characteristics, and it has been possible to produce elements which have a high degree of reliability.
A high temperature of at least 950xc2x0 C. is required to obtain a silicon oxide film which can be used as a gate insulating film by means of the thermal oxidation method, and there is no other substrate material apart from quartz which can withstand such high temperatures. A maximum process temperature of not more than 700xc2x0 C., and preferably of not more than 650xc2x0 C., is required if glass substrates which have a low strain point as described above are to be used, and it is impossible to satisfy this requirement with the thermal oxidation method.
Gate insulating films must be produced using physical gas phase growth (PVD) methods, such as the sputter method, or chemical gas phase growth (CVD) methods, such as the plasma CVD method and the thermal CVD method, because of these problems. A highest process temperature of not more than 650xc2x0 C. is a possibility with these methods.
However, insulating films which have been produced using the PVD methods and CVD methods have high concentrations of dangling bonds and hydrogen, and the boundary characteristics are not good. Consequently, they are weak in respect of the implantation of hot electrons, for example, and charge trapping centers are liable to be formed because of the presence of the dangling bonds and hydrogen. Consequently, when used as gate insulating films for TFT, there is a problem in that the electric field mobility and the sub-threshold characteristic value (S value) are not good, or there is a problem in that the leakage current of the gate electrode is considerable and the fall in the ON current (deterioration, change with the passage of time) is considerable.
The present invention provides a means of resolving the above mentioned problems. That is to say, the theme of the invention is to provide a method with which gate insulating films can be manufactured using crystalline silicon films with a thermal oxidation method of which the highest process temperature is not more than 700xc2x0 C.
In the present invention, a thermal oxide film is formed on the surface of a silicon film at a low temperature of 400-700xc2x0 C., and typically of 550-650xc2x0 C., by heat treating the silicon film in a specified atmosphere. In the present invention, the silicon film is thermally oxidized and a silicon oxide film is obtained by carrying out a thermal anneal at 400-700xc2x0 C. in a highly reactive atmosphere of oxygen or ozone, or nitrogen oxide (general formula NOx, where 0.5xe2x89xa6xc3x97xe2x89xa62.5), which contains thermally excited, or thermally decomposed, components. Dinitrogen monoxide (N2O), nitrogen monoxide (NO), nitrogen dioxide (NO2), or a mixture of these gases is preferred for the nitrogen oxide which is used when carrying out the thermal oxidation in the present invention.
The inclusion of hydrogen compounds such as water (H2O) in these atmospheres results in the inclusion of hydrogen in the thermal oxide films which are obtained and so this is undesirable. Similarly, the inclusion of carbon based gases (CO, CO2 and the like) is also undesirable. The concentration of water or carbon based gas in the atmosphere is preferably not more than 1 ppm, and most desirably not more than 10 ppb.
A gas which contains thermally excited or decomposed nitrogen oxide (or oxygen) is referred to hereinafter as reactive nitrogen oxide (or reactive oxygen). In the present invention, the reactive nitrogen oxide (reactive oxygen) may be comprised solely of nitrogen oxide (oxygen), or it may be admixed with argon or other inactive gases.
The upper limit of the oxidation temperature is determined by the type of substrate which is being used. Naturally, the oxidizing action proceeds more readily as the thermal oxidation temperature is increased. A temperature of 550-650xc2x0 C. is appropriate for a Corning 7059 substrate which is a typical glass substrate. In the present invention, the various glass substrates of which the strain temperature (strain point) is less than 750xc2x0 C., and typically 550-680xc2x0 C., as typified by Corning 7059 glass (alkali free boron silicate glass) should be used for the substrate.
An example of the apparatus for the execution of this invention is shown in FIGS. 1A and 1B. A first reaction chamber for thermally exciting the nitrogen oxide, or oxygen or ozone, initially and a second reaction chamber into which the reactive nitrogen oxide or oxygen obtained in the first reaction chamber is introduced for oxidizing the silicon film at a temperature of 400-700xc2x0 C. are required. In FIG. 1(A), 1 is the first reaction chamber and 5 is the second reaction chamber. These reaction chambers, and the connecting passageway 4 between them, must be maintained at appropriate temperatures. The heaters 2, 3 and 6 are provided for this purpose.
A temperature which is sufficiently high to render the nitrogen oxide reactive is required in the first reaction chamber. That is to say, it must be above the temperature at which the nitrogen oxide is decomposed. The optimum temperature depends on the type of gas, and with dinitrogen monoxide, for example, a temperature of 750-950xc2x0 C. is preferred. Furthermore, with oxygen a higher temperature of 1000-1200xc2x0 C. is required for thermal excitation and decomposition.
The gas molecules which are excited in the first reaction chamber revert to the ground state in cases where the temperature in the conduit 4 between the first and second reaction chambers is very low, and the reactivity is reduced. Hence, an appropriate temperature must also be maintained in the connecting passageway 4 in order to maintain the reactivity. The temperature in the connecting passageway 4 is preferably intermediate between those in the first and second reaction chambers. That is to say, if the temperature in the first reaction chamber 1 is TA, the temperature in the connecting passageway 4 is TB and the temperature in the second reaction chamber 5 is TC, then TAxe2x89xa7TBxe2x89xa7TC. Furthermore, it is desirable that the inner walls of the connecting passageway 4 should be made with a material of which main component is quartz so that it does not react with the reactive gas molecules. The use of quartz of a high purity, comprising at least 90 mol. % silicon oxide, is preferred.
If the inner wall is comprised of a metallic material, the atoms or excited molecules may revert to the ground state or be stabilized by recombining, and the reactivity is lost. However, in those cases where the inner walls are made of quartz this effect is slight and, for example, most of the atoms and molecules are still in an active state even at a distance of 50-100 cm from the first reaction chamber.
A plurality of substrates 8 are mounted on the susceptor 7 in the second reaction chamber 5 and a plurality of substrates can be treated at once. Since a high temperature gas flows from the first chamber into the second reaction chamber, it is necessary to increase the uniformity of the temperature distribution in the second reaction chamber 5 by optimizing the temperature and length of the connecting passageway 4. If there is a temperature distribution within the second reaction chamber it is difficult to treat a plurality of substrates at the same time in a uniform manner. Furthermore, reducing the pressure of the gaseous atmosphere below atmospheric pressure is also effective.
It is difficult to render most of the gas molecules in the first reaction chamber reactive in the example shown in FIG. 1A. This is because thermal energy required to excite or decompose the gas must be obtained from the walls of the reaction chamber, and only a part of the total number of gas molecules can contact with the wall in the first reaction chamber. More precisely, reactivity is achieved by means of the kinetic energy of other gas molecules, but the energy imparted is obtained directly from other gas molecules or indirectly from the walls of the reaction chamber. Of course, the invention can be carried out if just a few reactive molecules are present. Needless to say, however, the effect of the invention increases as the number of reactive molecules is increased.
A material 9 which conducts heat comparatively easily or which absorbs infrared radiation easily, such as a metal, can be placed within the first reaction chamber, as shown in FIG. 1B, in order to render more gas molecules reactive. Preferably, this material should be in a form such as a mesh which has a large surface area, which does not impede the flow of gas and which makes contact with a large amount of gas. Most desirably, the material 9 has a catalytic action. Examples of such materials include platinum, palladium, (reduced) nickel, titanium, vanadium and cobalt. The catalyst may have a powder-like form or a granular form instead of being in the form of a mesh.
When in contact with such a material, the gas molecules become reactive in the same way as when making contact with the walls of the reaction chamber, and more gas molecules become reactive as the surface area becomes greater. Moreover, it is possible to obtain even more reactive gas if such materials have a catalytic action. Furthermore, the provision of a temperature which is higher than that in the first reaction chamber by passing an electric current through the mesh-like metal 9 is also effective.
If a means such as those indicated above is adopted then it is possible to lower the temperature in the first reaction chamber as compared with the case shown in FIG. 1A.
Oxidation not just at 1 atmosphere (atmospheric pressure) but under a pressure of more than 1 atmosphere, but not exceeding 15 atmosphere, is also effective for increasing the oxidation rate. For example, an oxidation rate ten times higher is obtained at a pressure of 10 atmospheres when compared with oxidation at a pressure of 1 atmosphere. Furthermore, the oxidation temperature can also be reduced.
The relationship between the oxidation time and the thickness of the thermal oxide film which is obtained using the apparatus shown in FIG. 1 is shown in FIG. 5. Here dinitrogen monoxide was used for the oxidizing atmosphere. The oxidizing action proceeded more easily as the temperature was increased and as the pressure was increased.
Amorphous silicon films obtained by means of a CVD method such as a plasma CVD method or a reduced pressure CVD method are to be used as a starting material for forming a crystalline silicon film as an active layer in the present invention, and the methods of crystallization can be divided broadly into two types. The first method is that in which an amorphous silicon film is formed and then this is crystallized by thermal annealing at a temperature of 500-650xc2x0 C. for an appropriate period of time. Elements which promote the crystallization of amorphous silicon, such as nickel, iron, platinum, palladium and cobalt, may be added at the time of this crystallization. The crystallization temperature can be lowered and the crystallization time can be shortened if these elements are added.
The semiconductor characteristics of silicon are lost if these elements are included at high concentrations and so a low concentration which is enough for crystallization but which has virtually no effect on the semiconductor characteristics is preferred. That is to say, the minimum concentration in the silicon film as measured using secondary ion mass spectrometry (SIMS) is preferably 1xc3x971015xe2x88x923xc3x971019 atoms/cm3. The concentration distribution of such elements which promote crystallization varies according to the method of treating the silicon film and so there are cases where the minimum value is obtained at a boundary and cases where the minimum value is obtained in the middle of the film.
The second method is a laser annealing method in which amorphous silicon films are crystallized by being irradiated with strong light from a laser for example. Which of the two methods indicated above is chosen should be determined on the basis of the characteristics of the TFT for which the execution of the invention is required, the apparatus which can be used and the plant costs, for example.
Furthermore, the first and second methods may be combined with each other. For example, the crystallinity may be further increased by a laser anneal after the crystallization by the thermal annealing. In those cases where a crystallization promoting element such as nickel is added and thermal annealing is carried out in particular, amorphous parts have been observed to remain at the crystal grain boundaries, and the laser annealing method is effective for crystallizing such amorphous parts.
Conversely, the stress and strain in the film produced by laser annealing can be alleviated by thermally annealing the silicon film which has been crystallized by means of a laser anneal.
The thermal oxide films obtained by means of this invention can be used even as they are as gate insulating films, but subjecting the thermal oxide films obtained to a thermal anneal at 400-700xc2x0 C. in a hydrogen nitride atmosphere such as ammonia (NH3) or hydrazine (N2H4) is good for improving the characteristics even further. The dangling bonds in the silicon oxide film are completely taken up with nitrogen by means of this thermal anneal and some of the oxygen is replaced with nitrogen, and it is possible to form electrically stable silicon oxinitride films.
Furthermore, while only the thermal oxide film may be used as a gate insulating film, if the thermal oxide film does not reach the sufficient thickness, a further insulating may be formed over the thermal oxide film by a PVD method or a CVD method for example, and an insulating film which has a multi-layer structure can be used as a gate insulating film. Insulating films obtained by means of a PVD method or a CVD method, for example, generally have very poor characteristics when compared with a thermal oxide film, but the effect on the TFT characteristics is sufficiently small provided that the surface of the active layer is covered with the thermal oxide film having a thickness of at least 200 xc3x85.
A sputtering method can be used as a PVD method, and a plasma CVD, a reduced pressure CVD, and an atmospheric CVD method can be used as a CVD method, for example, for forming such insulating films in the present invention. Other methods of film formation can also be used. The plasma CVD methods and reduced pressure CVD method using TEOS as a raw material may be used. A substrate temperature of 200-500xc2x0 C. is preferred for depositing a silicon oxide film using TEOS and oxygen as raw materials in the plasma CVD method. Furthermore, the reaction in which TEOS and ozone are used in a reduced pressure CVD method proceeds at a comparatively low temperature (for example, at 375xc2x0 C.xc2x120xc2x0 C.), and silicon oxide films which are undamaged by the plasma can be obtained.
Similarly, silicon oxide films which are undamaged by a plasma can also be obtained with a reduced pressure CVD method using monosilane (SiH4) and oxygen (O2), or monosilane and dinitrogen monoxide, as raw materials.
The combination of monosilane and nitrogen oxide may be used in the plasma CVD method. Furthermore, in plasma CVD methods, the ECR-CVD method in which a discharge using an ECR (electron cyclotron resonance) condition is used causes little damage due to plasma and so it is possible to form even better gate insulating films with this method.
It was found by the inventors that insulating films which had silicon oxide which had been fortified to a certain extent was suitable for the gate insulating films of TFTs. Specifically, silicon oxide films of which the etching rate with a buffered hydrofluoric acid at 23xc2x0 C., containing hydrogen fluoride, ammonium fluoride and acetic acid at proportion of 1:50:50, was not more than 1000 xc3x85/minute, and typically 300-800 xc3x85/minute, were preferred. Silicon oxide films which contain, on average, 1xc3x971017xe2x88x921xc3x971021 atoms/cm3 of nitrogen mostly satisfy such an etching rate condition.
Insulating films which have been formed over a thermal oxide film in this way may be subjected to a thermal anneal in a dinitrogen monoxide atmosphere in order to improve their characteristics even further. This can be done at 300-700xc2x0 C. Furthermore, the annealing effect can be increased by irradiating the insulating film with ultraviolet light during the thermal anneal, and a similar effect can be achieved at an even lower temperature. This is because the dinitrogen monoxide is brought into an active state by the ultraviolet light and is able to react more easily with the insulating film which has been deposited using a CVD or PVD method.
Similarly, the insulating film may be treated by generating a plasma in an atmosphere which contains dinitrogen monoxide. Here again, the dinitrogen monoxide is excited and activated by the plasma and can react with the insulating film. Furthermore, it is necessary to reduce the dinitrogen monoxide pressure for generating the plasma on treatment with this plasma and so a thermal anneal may be carried out at 400-700xc2x0 C. in a dinitrogen monoxide atmosphere at a pressure of at least 0.1 atmosphere following the plasma treatment in order to complete the reaction more fully.
Atomic oxygen, or oxidizing molecules which have a similar reactivity to atomic oxygen, which has an oxidizing action must be formed in the atmosphere to make the oxidation reaction of silicon proceed. However, a very high temperature is required to obtain atomic oxygen for example from oxygen molecules. Consequently, thermal oxidation does not proceed in an atmosphere of dry oxygen unless the temperature is at least 1000xc2x0 C.
The present invention is based on the observation that under the appropriate conditions, the atomic oxygen and excited states of oxides of nitrogen obtained by heating oxides of nitrogen, oxygen or ozone and thermally exciting or decomposing them have a sufficiently long lifetime and that spatial movement is possible. That is to say, gas molecules or atoms which are heated to a high temperature and rendered reactive are introduced into a reaction chamber at a lower temperature and, as a result of their use, the thermal oxidation proceeds at a lower temperature than that in the conventional thermal oxidation method. In the present invention, the use of oxides of nitrogen or oxygen or ozone which is heated to a temperature above its decomposition temperature to obtain more atomic oxygen or oxides on nitrogen in an excited state is preferred.
In the present invention, the effects such as those indicated below also arise when a silicon film which is crystallized by means of a thermal anneal is used. Generally, if the gate insulating film and the active layer are thin, good characteristics in that the mobility is improved and the OFF current is reduced are obtained as they become thinner. On the other hand, the crystallization of the initial amorphous silicon film can be achieved more easily as the film thickness is increased. Hence, in the past there has been a conflict between the characteristics and the processing aspect in terms of the thickness of the active layer. This invention resolves this conflict first of all, which is to say that a thick amorphous silicon film is formed before crystallization and a silicon film which has good crystallinity is obtained, and then the silicon film is reduced in thickness by oxidation, and the characteristics as a TFT are improved. Moreover, in the thermal oxidation the amorphous components and crystal grain boundaries where recombination centers are liable to exist are oxidized readily and there is an advantage in that the number of recombination centers in the active layer is effectively reduced. Consequently, the product yield is also high.
The invention has a special effect when applied to active layers comprising crystalline silicon films which are crystallized with the addition of elements which promote the crystallization of amorphous silicon films, such as nickel, cobalt, iron, platinum and palladium for example. The crystallinity of a silicon film which is crystallized with the addition of such crystallization promoting elements is good, and films in which the electric field effect mobility is also very high can be obtained, but films which have good characteristics as gate insulating films are desirable as well. The gate insulating films obtained with the present invention are suitable for this purpose. Furthermore, the noncrystalline regions which remain at the crystal grain boundaries, for example, can also be crystallized during the thermal oxidation, and the crystallinity can be improved even further.
When the present invention is applied to an active layer in which a silicon film which is laser annealed is used, in addition to the effect of improving the characteristics of the gate insulating film, the annealing process of the present invention also has the effect of alleviating the strain in the silicon film caused by the laser annealing.
Furthermore, in those cases where a silicon film which has very good crystallinity, such as a mono-domain structure, is used, characteristics similar to those of a thermal oxide film are required for the gate insulating film of the present invention.